@Aeriandra : Or because Uri is such a 'nice guy' with inferiority complex that the enemy just had to play the pity card/'potentially a better candidate' card and he falters/falls for it, giving them the opening they need to get rid of their real targets: the father and Lamia, who can see the big picture enough to choose the 'cruel' option of killing the child to ensure peace in the south.
Given how much Lamia (and indirectly the father, since he never outright refuse it) ignore Uri being unfit for the Duke and still push him to be the heir anyway, I suppose those weakness for 'family' runs in the family. In this case it's just that the father/Lamia chose Uri over this other kid.
@Ihcky: I don't think he's a 'good' person as in a nice/benevolent type. If anything he's a person of extremes: The ones he care about he will spare no effort to protect (see: how he basically turned into sad puppy when Lamia get angry at him), but everyone else can go to hell for all he care.
People were confused at why he would attack Uri, even though it'll make Lamia mad at him (something he definitely doesn't want)
I at least wasn't expecting a 'good' reason, but a reasonable one. And anger over Uri's naivete leading to Lamia's death in the past + he still is being a naive bastard even after nearly getting killed definitely is enough for me.